1. Technical Field
The present invention relates to a toner for developing an electrostatic charge image and a process for its production. More specifically, it relates to a toner for developing an electrostatic charge image, which satisfies both low temperature fixing property and blocking resistance and which is excellent in the fixed image strength and further in the production stability and is capable of presenting a constant quality, and a process for its production.
2. Discussion of Background
The toner for developing an electrostatic charge image is used for formation of an image to visualize an electrostatic image in e.g. a printer, a copying machine or a facsimile machine. With reference to formation of an image by an electrophotographic system, an electrostatic latent image is firstly formed on a photoconductor drum, then it is developed with a toner and then transferred to e.g. transfer paper, followed by fixing by e.g. heat to form an image. The toner for developing the electrostatic charge image at that time is usually one in such a form that solid fine particles of e.g. silica are attached, as an additive, to the surface of toner particles obtained by a so-called melt-kneading pulverization method wherein to a binder resin and a colorant, as the case requires, an electrification-controlling agent, a release agent, a magnetic material, etc. are dry-mixed, followed by melt kneading by e.g. an extruder and then by pulverization and classification, for the purpose of imparting various properties such as flowability.
In recent years, in formation of an image by a copying machine or printer, a highly fine image quality is required, and in order to satisfy such a requirement, it is necessary that the average particle diameter of toner particles is at a level of from 3 to 8 μm, and the particle size distribution is narrow. However, in the melt kneading pulverization method, it is difficult to control the particle diameter of the toner particles, and if it is attempted to obtain toner particles having an average particle diameter within a range of from 3 to 8 μm, there has been a problem such that a fine powder having a diameter smaller than the desired particle diameter is formed in a large amount as a byproduct, and it has been difficult to separate it in a classification step.
As a method for overcoming such a problem in the melt kneading pulverization method, it has been proposed to use a production method by a polymerization method such as a suspension polymerization, an emulsion polymerization coagulation method or a solution suspension method, instead of the melt kneading pulverization method.
The suspension polymerization method is a method wherein a composition comprising a polymerizable monomer, a polymerization initiator, a colorant, etc. as components, is suspended and dispersed in an aqueous medium, followed by polymerization to obtain toner particles. The emulsion polymerization coagulation method is a method wherein a polymerizable monomer is emulsified in an aqueous medium containing a polymerization initiator and an emulsifier, the polymerizable monomer is polymerized with stirring to obtain polymer primer particles, to which a colorant and, as the case requires, an electrification-controlling agent, etc. are added to coagulate polymer primary particles, and further, the obtained coagulated particles are aged to produce toner particles. Further, the solution suspension method is a method wherein a binder resin is dissolved in an organic solvent, a colorant, etc. are added and dispersed to obtain a solution phase, which is dispersed in an aqueous phase containing a dispersant, etc. by a mechanical shearing force to form liquid droplets, and from such liquid droplets, the organic solvent is removed to produce toner particles.
By these polymerization methods, it is easy to control the particle diameters of the toner particles, and it is possible to obtain toner particles which have a small particle diameter and a narrow particle size distribution and which are capable of forming a highly fine image quality.
The suspension polymerization method and the emulsion polymerization coagulation method has a merit such that the energy required for the production of the toner is small as compared with the solution suspension method wherein granulation is carried out by means of a separately prepared binder resin, since the polymerization of the polymerizable monomer and the granulation of toner particles are carried out in the production process, and further it is thereby easy to prepare a toner having a small diameter and easy to control the particle size distribution or particle diameter.
Further, in recent years, along with dissemination of copying machines, printers, etc., in addition to the demand for the image quality, a toner excellent particularly in the high speed printing and low energy fixing property has been desired, and it has been attempted to improve the low temperature fixing property of the toner. The low temperature fixing property and the blocking resistance or high temperature offset resistance are usually in a trade-off relation, and it is difficult but desired to satisfy both properties.
To accomplish such an object, wax is used as an offset-preventing agent. However, the wax content in a toner is limited, and if wax is used excessively, leakage from the toner occurs to deteriorate the blocking resistance. Therefore, improvement of the low temperature fixing property by wax is limited.
As a method for improving the low temperature fixing property, a technique of incorporating a crystalline polyester resin to a non-crystalline resin thereby to improve the low temperature fixing property has been proposed (Patent Documents 1 to 5).
In a case where such a crystalline polyester resin is incorporated as dispersed in a non-crystalline resin having poor compatibility, for example, in a case where the non-crystalline resin is a styrene type resin, dispersed domains of the crystalline polyester component are not dispersed in a sufficiently small size, whereby the obtained toner is brittle as a drawback of the crystalline resin or exhibits adhesion to components during the development, or has had a problem such that the temperature range for fixing becomes very small, since the elasticity at the time of heating sharply decreases.
On the other hand, in a case where such a crystalline polyester resin is incorporated as dispersed in a non-crystalline resin having good compatibility, for example, in a case where the non-crystalline resin is a polyester resin, when it is dispersed by melt-kneading, no adequate dispersibility is obtainable, and it has been possible only to obtain a toner having the same drawback as in the case where dispersed in the non-crystalline resin having poor compatibility.
Further, if a crystalline polyester and a non-crystalline polyester are used in combination, such may be effective for improvement of the low temperature fixing property, but they become partly compatible, whereby the glass transition temperature lowers, and accordingly, the blocking resistance tends to be inadequate. With respect to this problem, it has been reported that an improvement is observed by carrying out a heat treatment step, but a very long time is required for the treatment.
In the case of mixing dispersions having such crystalline and non-crystalline polyester resins finely dispersed, respectively, too much energy or assisting power of an organic solvent is required to disperse the non-crystalline polyester resin in water, whereby the cost will be high, and if an alkali is used as a dispersing aid, there has been a problem such that the performance deteriorates due to hydrolysis. Further, a tin-type catalyst which has been commonly used to design the molecular weight so that this non-crystalline polyester resin will acquire a good fixing property, has a drawback of contaminating the environment, and a safe resin to assist a good fixing property has not yet been obtained.
Whereas, a method of using a low melting point crystalline resin containing a long chain (meth)acrylic acid ester has been proposed (Patent Document 6). Such a monomer can easily be emulsified and is very suitable for the production of a toner in an aqueous system. However, if such a long chain (meth)acrylic acid ester polymer is used as a binder resin, brittleness results, and the fixed image strength tends to be remarkably deteriorated, whereby image defects are likely to form by bending or scratching (Patent Document 6). Further, if a long chain (meth)acrylic acid ester and a vinyl monomer are simply copolymerized (Patent Documents 7 and 8), the melting point decreases, and the blocking resistance tends to be deteriorated. For the same reason, the blocking resistance tends to be deteriorated if a long chain (meth)acrylic acid ester polymer having a low melting point is used.
Further, a method for producing a latex has been proposed wherein a long chain (meth)acrylic acid ester polymer is modified by a non-crystalline resin to form a core/shell structure, and the long chain (meth)acrylic acid ester polymer is used as a release agent (Patent Document 9). However, such a latex has had a problem such that the storage elastic modulus is high in such a state that the long chain (meth)acrylic acid ester polymer is melted, and even if formed into a toner, the release effect is not sufficient only with the long chain (meth)acrylic acid ester polymer, and offset is likely to result, the gloss tends to be low, and further, since the particle diameter of the latex is large, coarse particles are likely to form when agglomerated together with a pigment.    Patent Document 1: US 2002/0106573 A1    Patent Document 2: US 2003/0040585 A1    Patent Document 3: JP-A-2005-234046    Patent Document 4: JP-A-2006-113473    Patent Document 5: US 2007/0207401 A1    Patent Document 6: U.S. Pat. No. 3,853,778    Patent Document 7: JP-A-7-301949    Patent Document 8: JP-A-8-95294    Patent Document 9: US 2005/0159530 A1